Epitaxial growth technique is conventionally used to produce a semiconductor device such as a power device (e.g., IGBT (Insulated Gate Bipolar Transistor)) requiring a relatively-thick crystalline film.
In the case of vapor phase epitaxy used in epitaxial growth technique, a wafer is placed inside a film-forming chamber maintained at atmospheric pressure or a reduced pressure, and a reaction gas is supplied into the film-forming chamber while the wafer is heated. As a result, a pyrolytic reaction or a hydrogen reduction reaction of the reaction gas occurs on the surface of the wafer so that an epitaxial film is formed on the wafer.
In order to produce a thick epitaxial film in high yield, a fresh reaction gas needs to be continuously brought into contact with the surface of a uniformly-heated wafer to increase a film-forming rate. Therefore, in the case of a conventional film-forming apparatus, a film is epitaxially grown on a wafer while the wafer is rotated at a high speed (see, for example, Japanese Patent Application Laid-Open No. 2008-108983).
In order to form a silicon (Si) epitaxial film on a wafer, a mixture gas obtained by mixing a silicon (Si) source gas such as silane (SiH4) or dichlorosilane (SiH2Cl2), a small amount of dopant gas such as diborane (B2H6) and phosphine (PH3), and a carrier gas such as hydrogen (H2) gas is used as the reaction gas.
In recent years, attention has been given to SiC (silicon carbide) epitaxial growth technique. SiC is characterized in that its energy gap is two or three times larger and its dielectric breakdown field is about one digit larger than that of a conventional semiconductor material such as Si (silicon) or GaAs (gallium arsenide). For this reason, SiC is a semiconductor material expected to be used in high-voltage power semiconductor devices.
In order to obtain a SiC single-crystalline thin film by epitaxial growth of SiC, a mixture gas obtained by mixing, for example, a silicon (Si) source gas such as silane (SiH4) or dichlorosilane (SiH2Cl2), a carbon source gas such as propane (C3H8) or acetylene (C2H2), and a carrier gas such as hydrogen (H2) gas, is used as the reaction gas. Further, in order to form a SiC film on a substrate by epitaxial growth, the substrate needs to be heated to a temperature of 1500° C. or higher. Therefore, a heater for heating a wafer is made of a high heat-resistance material. More specifically, a heater obtained by coating a carbon (C) base material with SiC by CVD (Chemical Vapor Deposition) is used.
It is known that a reaction between SiC and hydrogen gas occurs under high temperature. Therefore, if the reaction gas comes into contact with the heater during epitaxial growth of SiC, SiC that covers the surface of the heater reacts with hydrogen gas contained in the reaction gas and is decomposed. As a result, carbon as a base material of the heater is exposed and reacts with hydrogen gas according to the following equation:C+2H2→CH4 Such a reaction between the base material of the heater of the film-forming apparatus and hydrogen gas degrades and shortens the lifetime of the heater.
In view of the above problem, it is an object of the present invention to provide a film-forming apparatus that can prolong the lifetime of a heater and can be used under high temperature, and a film-forming method that makes it possible to prolong the lifetime of a heater and to perform film formation under high temperature.
Other challenges and advantages of the present invention are apparent from the following description.